Gain-producing, large-mode-area, multimode, hybrid optical fibers and devices using same

Abstract

A large mode area, gain-producing optical fiber is configured to support multiple transverse modes of signal radiation within its core region. The fiber is a hybrid design that includes at least two axial segments having different characteristics. In a first axial segment the transverse refractive index profile inside the core is not radially uniform being characterized by a radial dip in refractive index. The first segment supports more than one transverse mode. In a second axial segment the transverse refractive index profile inside the core is more uniform than that of the first segment. The two segments are adiabatically coupled to one another. Illustratively, the second segment is a terminal portion of the fiber which facilitates coupling to other components. In one embodiment, in the first segment M12>1.0, and in the second segment M22<>1.0 and M22˜1.0.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Our invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic block diagram of a prior art REDFA;

FIG. 2 is a schematic of an optical fiber 12′ showing an input segment 12i, an adiabatic coupling segment 12a, and a low M² terminal segment 12t, in accordance with one embodiment of our invention;

FIG. 3 is a schematic of an optical fiber 12″ showing, in addition, another low M² segment 12m disposed between a pair of adiabatic coupling segments 12a₂ and 12a₃, in accordance with another embodiment of our invention;

FIG. 4A is a schematic cross-sectional view of an REDF taken through its axis of propagation;

FIG. 4B is a schematic transverse refractive index profile of a terminal segment of the REDF shown in FIG. 4A, in accordance with yet another embodiment of our invention;

FIG. 4C is a schematic transverse refractive index profile of an input segment of the REDF shown in FIG. 4A, showing a pronounced dip in the profile at or near the center of the core region, in accordance with still another embodiment of our invention;

FIG. 4D is an expanded view of the pronounced dip in the schematic refractive index profile of the core region of FIG. 4C;

FIG. 5 is a graph of the core-cladding transverse refractive index step (Δn) versus radial position for an as-drawn fiber (Curve 5.1), a heat-treated fiber (Curve 5.2), and a uniform step index fiber (Curve 5.3);

FIG. 6 is a graph of normalized optical intensity of the fundamental transverse mode (LP₀₁) for an as-drawn fiber (Curve 6.1), a heat-treated fiber (Curve 6.2), and a uniform step index fiber (Curve 6.3); and

FIG. 7 is a graph of normalized optical intensity for a fiber with a pronounced center dip and for a uniform step index fiber versus radial position comparing the overlap between the fundamental transverse mode and the index profile of the as-drawn fiber (Curve 7.1) and the ideal uniform step index fiber (Curve 7.3) of the fiber designs of FIG. 5.

Claims

1. A multi-transverse-mode rare-earth-doped optical fiber comprising: a core region doped with at least one rare earth element, the cross-section of said core region having a transverse refractive index profile,a cladding region adjacent said core region,said core and cladding regions configured to support multiple transverse modes of optical signal radiation within said core region,said fiber including a first axial segment in which said profile is not radially uniform being characterized by a radial dip in refractive index, said first segment supporting more than one of said transverse modes,said fiber having a second axial segment optically coupled to said first segment, said profile of said second segment being more uniform than that of said first segment, andsaid segments being adiabatically coupled to one another.

2. The fiber of claim 1, wherein said first segment is characterized by a parameter M12 and said second segment is characterized by a parameter M22, where M2 defines the similarity that the fundamental transverse mode of said fiber has to an ideal Gaussian function, and wherein M12>1.0 and M22<<M12.

3. The fiber of claim 2, wherein M12>>1.0 and M22˜1.0.

4. The fiber of claim 1, wherein said first segment comprises a major portion of the length of said fiber, and said second segment comprises a terminal portion of said fiber.

5. The fiber of claim 4, wherein said fiber includes a third axial segment optically coupled to said first segment, said profile of said third segment being more uniform than that of said first segment and being adiabatically coupled to said first segment, said second segment being located at one end of said first segment and said third segment being located at the opposite end of said first segment.

6. The fiber of claim 1, wherein said profile of said core region exhibits a dip in refractive index of Δnd, which is approximately 5-100% of the difference Δn in transverse refractive index between said core region and said cladding region.

7. The fiber of claim 1, wherein fiber is configured to propagate said signal radiation in the fundamental transverse mode.

8. The fiber of claim 1, wherein said first segment comprises a major portion of the length of said fiber, and said second segment comprises an intermediate portion of said fiber.

9. The fiber of claim 1, wherein said core and cladding regions are configured to form a large mode area fiber.

10. An optical amplifier comprising: an optical fiber according to claim 1 for amplifying said signal radiation in response to optical pump energy applied thereto,a source of said pump energy, anda coupler for coupling said pump energy and said optical signal into said optical fiber.

11. The amplifier of claim 10, wherein said optical signal has a first center wavelength and said source of pump energy comprises a semiconductor light source for generating an optical pump signal having a second center wavelength.

12. A high power optical amplifier comprising: a multi-transverse-mode, large-mode-area hybrid optical fiber including a core region doped with at least one rare earth element, the cross-section of said core region having a transverse refractive index profile,said core region configured to amplify an optical input signal propagating therein in response to optical pump energy applied thereto,a cladding region adjacent said core region,said core and cladding regions configured to support multiple transverse modes of optical radiation within said core region,said fiber including a first axial segment in which said profile is not radially uniform being characterized by a radial dip in refractive index, said first segment supporting more than one of said transverse modes,said fiber having a second axial segment optically coupled to said first segment, said profile of said second segment being more uniform than that of said first segment,said segments being adiabatically coupled to one another so that energy propagating in particular transverse mode in said first segment is not significantly coupled into other transverse modes in said second segment; andsaid first segment being characterized by a parameter M12 and said second segment being characterized by a parameter M22, where M2 defines the similarity that the fundamental transverse mode of said fiber has to an ideal Gaussian function, and wherein M12>1.0 and M22<<M12, said second segment being located at either an input end of said first segment, at an output end of said first segment, or both,a LED for generating said optical pump energy at a center wavelength different from that of said optical signal, anda pump combiner for coupling said pump energy into said fiber.

13. The amplifier of claim 12, wherein M12>>1.0 and M22˜1.0.

14. The amplifier of claim 12, wherein said profile of said core region exhibits a dip in refractive index of Δnd, which is approximately 5-100% of the difference Δn in transverse refractive index between said core region and said cladding region

15. A multi-transverse-mode optical fiber comprising: first and second fiber segments each exhibiting optical gain, each segment having a core region and a cladding region adjacent said core region, the cross-section of each of said core regions having a transverse refractive index profile,said core and cladding regions configured to support multiple transverse modes of optical signal radiation within said core regions,said profile within said first axial segment not being radially uniform and being characterized by a radial dip in refractive index, said first segment supporting more than one of said transverse modes,said second axial segment optically coupled to said first segment, said profile of said second segment being more uniform than that of said first segment, andsaid segments being adiabatically coupled to one another.

16. The fiber of claim 15, further including a third segment axially disposed between said first and second segments, said third segment being configured to adiabatically couple said first and second segments to one another.

17. The fiber of claim 16, wherein said third segment does not exhibit optical gain.

18. The fiber of claim 17, wherein said first and second segments are rare-earth-doped and said third segment is not.